During the 110th Congress, the Democratic leadership of the Senate specifically blocked Republican President George W. Bush from making any recess appointments with the use of pro forma sessions.
In 2012 President Barack Obama attempted to make four appointments during a pro forma session, calling the practice of blocking recess appointments into question. However, on June 26, 2014 the Supreme Court of the United States determined that the President had improperly used his presidential power to make these appointments stating that while the Senate was in recess punctuated by pro forma sessions the period of time between the sessions was not long enough to invoke such power.
Like it or not, that trick's specifically enshrined and established in the Constitution. That same document Scalia claimed to defend to vigilantly. Probably there for good reasons.
Again, I'm hardly a scholar of the US Constitution, but if the excerpt of the Federalist papers by Hamilton quoted by Wikipedia[1] is anything to go by, the reason is very clear: to allow for an appointment which "might be necessary for the public service to fill without delay" while the Senate is in recess. That it allows for this trick seems like an unintended consequence.
And if the Senate itself is engineering that delay?
See also rules for allowing legislation to become law, or deny same, without explicit action. Pocket veto and default approval can both happen in this manner:
It's the only way they can try to run out the clock on Obama.
Or come up with some kind of fake moral outrage.
Can you imagine Trump picking the next three Supreme Court judges? It would set back America for decades.